The present invention deals with a process for the manufacture of cellulosic moulded bodies, in particular cellulosic flat films and cellulosic membranes in the form of flat membranes whereby a solution of cellulose in an aqueous tertiary amine oxide is extruded by means of an extrusion nozzle, which has an extrusion gap, whereby the solution is formed in the shape of a film and said solution is led via an air gap into a precipitation bath.
From U.S. Pat. No. 2,179,181 it is known that tertiary amine oxides have the ability to dissolve cellulose and that cellulosic moulded bodies such as fibres can be won from these solutions as a result of precipitation. A process for the production of solutions of this kind is for example known from EP-A-0 356 419. According to this publication first of all a suspension of cellulose is prepared in an aqueous tertiary amine oxide. The amine oxide contains up to 40 weight % water. The aqueous cellulose suspension is heated and whilst subjected to a reduction in pressure water is drawn off until such time as the cellulose dissolves.
From DE-A-28 44 163 it is known for the production of cellulose fibres that an air passage respectively air gap is provided between the spinning nozzle and precipitation bath to achieve drawing at the nozzle. This nozzle drawing is necessary since the stretching of the filaments is made more difficult after the contact of the moulded spinning solution with the aqueous precipitation bath. The fibre structure set in the air gap is fixed in the precipitation bath.
A process for the production of cellulosic threads is, furthermore, known from DE-A-28 30 685 whereby a solution of cellulose is formed to filaments in a tertiary amine oxide in a warm condition, the filaments are cooled down with air and finally introduced to a precipitation bath to precipitate the dissolved cellulose. The surface of said spun threads is, furthermore, moistened with water to reduce their tendency to stick to neighboring threads.
A device and a process for the production of seamless tubular films is known from WO 93/13670. According to this well-known process the cellulose solution is formed to a tube by means of an extrusion nozzle with a ring-shaped extrusion gap which is drawn over a cylindrical mandrel and introduced to the precipitation bath. So that the extruded tube does not remain sticking to the surface of the mandrel, its surface is covered with a film of water so that the inside of the tube coagulates and glides across the cylindrical mandrel. The tubular films obtained have wet strengths of about 10 to 30 N/mm.sup.2.
WO 95/07811 also describes a device and a process for the production of cellulosic tubular films.
WO 95/35340 describes a blowing process for the production of oriented cellulosic films whereby the cellulose solution is extruded downwards into a precipitation bath via a film blowing nozzle and an air gap. It is mentioned that stretching can be achieved transverse to the transport direction of the blown film via the gas pressure in the inside of the blown film and that the ratio of mechanical longitudinal to transverse properties can be set.
From EP-A - 0 042 517 the production of dialysis membranes is known by extruding the cellulose solution using nozzles with a length of 18 cm.
From DE-A-195 15 137 it is known that when producing cellulosic flat films using grooved nozzles the extruded film undergoes considerable width shrinkage in the air gap. This is undesirable and leads to significant fluctuations in film thickness of up to 150% apart from an insufficient film width. For this reason, for the production of flat films DE-A - 195 15 137 suggests first of all producing tubular films using a ring nozzle which are then cut to flat films. A special film blowing device is used to produce the tubular films with which the extruded tube is stretched in the air gap both in the draw-off direction and in the transverse direction, This happens as a result of gas pressure effective in the inside of the tube which extends the tube. One disadvantage of this process lies in the complicated design of the device to be used.
Moreover, on the other hand, the expert in the field of the extrusion of thermoplastic polymers, such as polyethylene or polypropylene, knows that films with a much better distribution of thickness transverse to the direction of extrusion can be achieved with flat nozzles rather than with round nozzles.
With cellulosic membranes particularly in the form of flat membranes, i.e. membranes from a flat film, the permeability of the membranes is an important property. To solve certain separation tasks it is important to select membranes with the optimum permeability, pore size and pore structure for the respective separation task.
Dialysis membranes made of regenerated cellulose in the form of flat films, tubular films or hollow threads have been known for some time whereby the regeneration of the cellulose can take place by means of the cuoxam-process, the viscose process or by means of the hydrolysis of cellulose acetate. Depending upon the process used and the process conditions one obtains membranes with different dialysis properties.
U.S. Pat. No. 4,354,938 describes for example a process for the production of dialysis membranes according to the viscose process, in which a tubular moulded membrane is stretched in the transverse direction by between 40 and 120% by blowing up with air before drying, which leads to a membrane with a regular orientation in the longitudinal and transverse direction. When transferring the dried membranes in the wet state the membranes thus produced undergo a shrinkage in the longitudinal and transverse direction of 0.5-10%. The ultrafiltration values lie in the range of between 2.5 ml/m.sup.2.h.mm Hg and 5.2 ml/m.sup.2.h.mm Hg at a wet thickness of 184 .mu.m to 45 .mu.m.
In "membranes and membrane processes" by E. Staude, 1992, VCH Verlagsges.m.b.H. on page 19 it is described that the biaxial stretching of finished cellophane membranes lead to the enlargement of the pores, monoaxial stretching on the other hand leads to a reduction in the effective pore diameter.
However, the viscose process only offers limited possibilities to set membrane properties in a well aimed manner. Moreover, the recovery of the chemicals which accumulate in this process such as sodium sulphate and carbon bisulphide etc. is very expensive.